Apparatus are known for stabilizing various well tools which are suspended at the bottom of a production tubing string. An example of a tool which would benefit from stabilization is a progressive cavity pump ("PC pump"). A PC pump is located within an oil well, positioned at the bottom end of a production string which extends down the casing of the well. The pump pressurizes well fluids and drives the up the bore of the production string to the surface. The pump comprises a pump stator coupled to the production tubing string, and a rotor which is both suspended and rotationally driven by a sucker rod string extending through the production string bore. The stator is held from reactive rotation by a tool anchored against the casing. Usually this anti-reactive, or no-turn tool is located at the base of the stator. Typically a no-turn tool applies serrated slips to grip against the casing.
The rotor is a helical element which rotates within a corresponding helical passage in the stator. Characteristically, the rotor does not rotate concentrically within the stator but instead scribes a circular or elliptical path. This causes vibration and oscillation of the sucker rod, the pump's stator and the tubing attached thereto.
The greater the pump flow, the greater is the vibration. This can lead to loosening of the slips and functional failure of the no-turn tool. Other problems include fatigue failure of the connection of the stator to the tubing or nearby tubing-to-tubing connections.
In the prior art, bow springs have typically been used to centralize and stabilize the stator and the supporting tubing. By design, the bow springs are radially flexible, in part to permit installation and removal through casing. Unfortunately, the spring's flexibility permits cyclic movement, resulting in fatigue and eventual failure of the springs.
Unitary tubing string centralizers generally position the tool in a concentric or central position in the well. While these centralizers may provide positioning function, they are not effective as a tool-stabilizing means. The known centralizers are passive devices and do not actively contact the casing.
More sophisticated apparatus are known which more positively secure and position tools within a well. For example, in U.S. Pat. No. 2,490,350 to Grable, a centralizer is provided using mechanical linkages which lock radially outwardly to engage the casing. Each of a plurality of two-bar linkages is held tight to the outside of the tubing string with a retaining bolt. A longitudinal spring and longitudinal ratchet are arranged external to the tubing for pre-loading of one link with the potential to jack-knife the linkage outwardly, except for the restraining action of the retaining bolt. A radial plunger extends through the tubing wall to contact the linkage. The plunger has limited stroke. When the tubing string bore is pressurized, the plunger urges the linkage sufficiently outwardly to break the retaining bolt, permitting the spring to drive the linkage radially outwardly. The driven link engages the ratchet, ensuring the linkage movement is uni-directional.
In U.S. Pat. No. 4,960,173 to Cognevich, a tubular housing is also disclosed having mechanical linkages which are held tight to the housing during installation. The linkages are irreversibly deployed upon melting of a fusible link at downhole conditions. An annular compression spring actuates a telescoping sleeve which deploys a four-bar linkage and forcibly holds the linkage against the casing wall. Rollers on the ends of two of the linkages contact the casing wall for aiding in limited longitudinal movement of the tubular housing once the linkages are deployed. Gradual radial adjustment of the linkage is permitted by a fluid bleed to permit the telescoping sleeve to slowly retract during this movement. If the bleed fails and additional radial movement is continues, a pin will shear, fully releasing the telescoping sleeve and linkage from the compression spring.
In summary, both Grable and Cognevitch disclose apparatus which:
rely upon compression spring force alone to drive and hold the linkages radially outwardly; PA1 do not deploy or extend the linkage until after installation on the casing; PA1 result in an irreversible deployment; and PA1 in the case of Grable do not permit movement or removal without damage to the linkage, and in the case of Cognevitch, limited movement is permitted but if the linkage cannot accept the movement required, a jarring action will shear a pin and irreversibly separate the compression spring from the linkage.
Therefore, for well tools which require secure stabilization within the casing, there is a demonstrated for a device which is capable of providing a stabilizing force which is greater than that provided by spring force alone, yet is still capable of being moved within the casing without irreversible damage to the apparatus.